Signal delay in metal interconnect structures depends on a time constant known as an RC delay, which is the product of the resistance of a metal line through which electrical current flows and the capacitance between the metal line and neighboring conductive structures. The capacitance is proportional to the effective dielectric constant of the dielectric materials between the metal line and the neighboring conductive structures and the effective area of the capacitive structure including the metal line. Moreover, the capacitance is inversely proportional to the effective distance between the metal line and the neighboring conductive structures.
The effective area of the capacitive structure and the effective distance between the metal line are geometrical factors that depend on the design of a metal interconnect structure. The effective dielectric constant can be decreased by employing materials having a low dielectric constant. For example, advanced semiconductor chips designed for high performance employ low dielectric constant (low-k) materials having a dielectric constant less than 3.9, which is the dielectric constant of silicon oxide. Such low-k dielectric materials include organosilicate glass (OSG) and low-k spin-on materials.
Formation of air cavities between metal lines can decrease the effective dielectric constant of a capacitive structure further. For example, U.S. Patent Application Publication No. 2009/0200636 discloses a cavity having a dielectric constant of 1.0 that is embedded between neighboring metal lines. Such air cavities, however, can increase time dependent intralevel dielectric failure by providing a surface diffusion path for impurities along the surface of a cavity. Further, a misalignment during a lithographic step employed to pattern a cavity can result in placement of sidewalls of metal lines in proximity to a cavity surface to exacerbate the diffusion of impurities along the surface of the cavity. Thus, the unintended side effect of a cavity can be deterioration of electrical reliability between a neighboring pair of metal lines around the cavity.